Diamond has captured the attention of scientists for a long time because of its extreme properties. It is the hardest known material (˜90 GPa), and until recently was considered to have the highest bulk modulus (1.2×1012 N/m2) and lowest compressibility. It has the highest known thermal conductivity at room temperature (2×103 W/m·K) and an extremely low thermal expansion coefficient (0.8×10−6 K−1). It is characterized by a broad optical transparency in the UV and IR regions. It is an electric insulator with a resistivity at room temperature of ˜1016 Ω·cm, which may vary over a range as large as 10–1016 Ω cm when it is doped. It is biocompatible, and exhibits negative electron affinity. See May, P. W., CVD Diamond—A New Technology for the Future, Endeavour Magazine 1995, 19(3), pp 101–106.
The first synthetic diamond crystals were produced from graphite using very high pressures and temperatures during the early 1950s, independently by two research teams, one in Sweden (Allmänna Svenska Elektriska Aktiebolaget) and the other in the USA (General Electric Company). See Ball, P., Hard Work: Diamond and Hard Materials, In Made to Measure: New Materials for the 21st Century, Princeton University Press, 1999, p 313. During the subsequent decades the high pressure, high temperature GE diamond producing technology attained commercial status and has now become available world-wide.
The interest in diamond has once again resurged due to research demonstrating that polycrystalline diamond films and diamond-like carbon films can be deposited at low pressures and at relatively low temperatures by using techniques such as Chemical Vapor Deposition (CVD). Presently, all CVD approaches employed for the deposition of diamond films require the presence of activated carbon-containing precursors. Other methods for the synthesis of diamond films have involved the use of thermal, plasma, and combustion-flame approaches.
Most of the diamond and diamond-like carbon (DLC) films have been synthesized by starting with hydrocarbon (HC) precursors such as methane and acetylene. Studies have demonstrated that even if graphite, and not diamond, is the thermodynamically stable solid carbon allotrope, diamond and diamond-like structures could be produced at low pressures and temperatures in the presence of hydrogen atoms. See Landis, C.; Cleveland, T.; Cloninger, M. J.; and Pollock, D., Buckyballs, Diamond, and Graphite, In Topic Oriented Approach Development, available online at www.chem.wisc.edu/˜newtrad/CurrRef/TOAHome/TOAHome; Davis, R. F. Ed. Diamond Films and Coatings, Noyes Publications, 1993; and Monaghan, D. P.; Laing, K. C.; Logan, P. A.; Teer, P. and Teer, D. G., How to deposit DLC successfully, Materials World 1993, 1(6), pp 347–349. The role of hydrogen atoms in the formation of diamond and DLC structures has been attributed to a number of factors. These include, (i) their efficiency for abstracting hydrogen atoms from the precursor hydrocarbon molecules, thereby generating an “activated” carbon-based species, (ii) their ability to “neutralize” surface dangling bonds thereby preventing cross-linking reaction mechanisms which are responsible for the formation of graphitic structures; and (iii) their etching specificity under certain experimental conditions, which provides preference for diamond formation at the expense of graphite.
During CVD and Plasma Enhanced CVD (PECVD) processes, the activated molecular fragments resulting from a specific HC/H2 mixture are deposited on substrate surfaces at 600–900° C. These high temperatures are usually required to shift the reaction equilibrium towards the formation of diamond structures. However, the high substrate temperatures considerably limit the applications of these films because they cannot be deposited on temperature-sensitive substrates, such as polymers and low melting point alloys. Moreover, CVD techniques result in deposition of films composed of a network of small diamond crystals in the micrometer range, which may be unsuitable for applications requiring highly uniform and smooth films.